Replace the entire header

I’m using Minimosity theme from themeforest on my blog. I tried to change the text logo at the header of the site. But, I found only a way to replace the text logo from the header. But, I’d like to change the entire header. I want to upload the header designed by my self. I need to do all this without disturbing the menu. How can I do this? I need your assistance.

Thanks.